The use of molds for sagging glass articles has been well known in the prior art, however such know molds have not been entirely satisfactory due to the cost and complexity of manufacturing the same, and further due to the undesirable physical characteristics exhibited thereby during heat-up and cool-down. That is, metal mold materials generally are not easily formed to exact contours and normally exhibit high thermal expansions which result in the warping of the mold during the hot temperature thermal cycling required for sagging glass, and accordingly an accurate work surface cannot be maintained. Further, the high thermal expansion not only tends to distort the precise curvature formed on the mold surface, but also laterally displaces the desired curvature relative to the glass article being sagged thereon thus producing dimensional problems in the glass being formed. In addition to the problem of high thermal expansion, metals have the further disadvantages of having a relatively low mechanical strength to thickness ratio at required sagging temperatures, the surface has a tendency to oxidize and deteriorate at such temperatures, and expensive finishing operations are required to generate plane or curved surfaces.
Ceramic molds of conventional materials do not provide a durable smooth surface required for sagging, since such molds tend to spall with thermal cycling. In addition, such refractory materials must be of a substantial thickness in order to provide the required strength and handling characteristics. Such thickness requirements, however, necessitate the use of longer thermal cycling periods in view of the heat absorbed by the ceramic mass of the mold.
Accordingly, the present invention has overcome the problems of thermal expansion, surface deterioration, low mechanical strength to thickness ratios, and complex machining requirements associated with the molds of the prior art, by providing an improved mold material which is not only easily formable to a desired surface contour, but which has a low thermal expansion. Further, an improved method of forming a plurality of such molds having virtually identical surface contours is set forth which overcomes the manufacturing problems heretofore encountered in the prior art.